The present invention relates to the art of magnetic field gradient generation. It finds particular application in conjunction with establishing gradient magnetic fields in magnetic resonance imaging techniques and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in spectroscopy and other processes and apparatus in which accurately predictable magnetic field gradients are established or maintained.
In magnetic resonance imaging, a uniform magnetic field is created through an examination region in which a subject to be examined is disposed. A series of radio frequency pulses and magnetic field gradients are applied to the examination region to excite and manipulate magnetic resonances. Gradient fields are conventionally applied as a series of gradient pulses with preselected profiles.
The gradient magnetic pulses are applied to select and encode the magnetic resonance signals. In some cases, the magnetic field gradients are applied to select one or more planes or slices to be imaged. Gradient field pulses are also applied for selectively modifying the uniform magnetic field to encode frequency and phase into the magnetization, hence the resonance signals in order to identify a spatial location.
The magnetic resonance signals are then processed to generate two or three dimensional image representations of a portion of the subject in the examination region. The accuracy of the resultant image representation, among other factors, is dependent upon the accuracy with which the actually applied magnetic field gradient pulses conform to selected gradient pulse profiles.
Conventionally, the uniform main magnetic field is generated in one of two ways. The first method employs a cylindrically shaped solenoidal main magnet. The central bore of the main magnet defines the examination region in which a horizontally directed main magnetic field is generated. The second method, employs a main magnet having opposing poles arranged facing one another to define the examination region therebetween. The poles are typically connected by a C-shaped or a four post ferrous flux return path. This configuration generates a vertically directed main magnetic field within the examination region. The C-shaped main magnet, often referred to as having open magnet geometry, has been able to resolve important MRI issues such as, increasing the patient aperture, avoiding the patient claustrophobia, and improving access for interventional MRI applications. However, the design of gradient coils for generating linear magnetic field gradients differs from that for the cylindrical type horizontal field system due to the direction of the main magnetic field.
In the solenoidal coil type systems, conventional gradient coils included coils wound in a bunched or distributed fashion on a large diameter hollow right cylinder tube. Conventional bunch geometries include Maxwell or modified Maxwell pair for Z gradient production and single or multi-arch golay saddle coils for X and Y gradient production. The coils are normally wound in a series arrangement in a position to give a magnetic field profile with the desired linearity over a predefined volume. However, the large inductances which are typical in wound cylindrical gradient coils, limit the switching speed of the gradient magnetic field.
Planar gradient magnetic field assemblies have been developed for the cylindrical main magnets with horizontally directed fields. The assembly includes a pair of planar Y-gradient coils, a pair of planar X-gradient coils, and a pair of Z-gradient coils. However, this type of planar gradient magnetic field assembly would not be directly compatible with main magnets having vertically directed fields. That is to say, that in main magnets with horizontally directed fields the direction of the main magnetic field is parallel to the planar surface of the gradient coil while in main magnets with vertically directed fields the direction of the main field is orthogonal or transverse to the planar surface of the gradient coil.
In open magnet systems with vertically directed fields, it has been known to use a bi-planar gradient coil system for generation of the magnetic field gradients. However, the use of this type of bi-planar gradient coil system somewhat detracts from the purpose of using an open magnet geometry in that it reduces the patient aperture and diminishes access for interventional MRI applications by employing two planar gradient coils one on either side of the subject being examined. As well, the performance of the bi-planar configuration can suffer in terms of its strength and slew rate.
The present invention contemplates a new and improved gradient coil configuration is provided which overcomes the above-referenced problems and others.